Add support for NSURLSession

Adds support for stubbing requests made using `NSURLSession`.
Updates AFNetworking in the UnitTests to v2.0
Unit test for `AFHTTPSessionManager`

@AliSoftware The one caveat is that the entire static lib must be built to target 7.0+ for my changes to be compiled. Alternatively it can be compiled directly from source in a parent project that targets 7.0+.

Please let me know if you'd prefer if that were handled differently.